FIG. 25 is a circuit diagram showing an exemplary configuration of a conventional voltage controlled oscillator (hereinafter, abbreviated as a “VCO”) having a frequency modulation function used in communication equipment.
In FIG. 25, reference numeral 5 denotes an output circuit, 11, 12, 13, and 14 denote varactor diodes, 16 and 17 denote inductors, and 26 denotes a voltage source.
An anode side of the varactor diode 13 and an anode side of the varactor diode 14 are connected to a voltage input terminal, and an anode side of the varactor diode 11 and an anode side of the varactor diode 12 are connected to a modulation signal terminal. A cathode side of the varactor diode 11 and a cathode side of the varactor diode 13 are connected to one end of the inductor 16, and a cathode side of the varactor diode 12 and a cathode side of the varactor diode 14 are connected to one end of the inductor 17. The other ends of the inductor 16 and the inductor 17 are connected to the voltage source 26. The resonance of the inductors and the varactor diodes realizes the VCO.
Frequency modulation is performed by changing a capacitance value of the varactor diodes 11 and 12 by inputting a voltage to the modulation signal terminal.
In the conventional VCO, when the inductors or the varactor diodes vary in inductance or capacitance, the input voltage versus oscillation frequency characteristics (hereinafter, abbreviated as “Kv”) of the VCO are changed. When a modulation voltage is input from the modulation signal terminal at a constant amplitude in this state, a modulation factor of an output signal is changed in accordance with the variations of Kv. In order to obtain a constant modulation factor, it is necessary to configure the VCO such that the inductors are provided as external components so as to allow the inductance to be adjusted, or that discrete components having little variation are used as the varactor diodes. Further, the voltage versus capacitance characteristics of the varactor diodes are not constant, and their nonlinearity also causes a modulation factor to be changed. Consequently, it is difficult to use the VCO over a wide oscillation frequency range.
In recent years, the downsizing of communication equipment has been required, and accordingly there is a need to incorporate a VCO into an IC. In the case of incorporating inductors and varactor diodes in the IC, element variations are larger than in the case where discrete components are used. Accordingly, the VCO is required to have means for compensating a required oscillation frequency range or variations of a modulation factor at the time of frequency modulation.
In conventional VCO circuits, there is no relationship between a voltage for determining an oscillation frequency of the VCOs in a voltage input terminal and a control signal amplitude for determining a modulation factor in a modulation signal terminal, which makes it difficult to compensate variations of a modulation factor. In conventional VCOs with a frequency modulation function, there is no relationship between a voltage input terminal for determining an oscillation frequency of the VCOs and a modulation signal terminal for performing frequency modulation, and they are controlled individually as respective circuits.